The attempt to differentiate social behavior on the basis of organic and genetic endowment is

2 Estimation of Heritability

The genetic influence on temperamental differences in a human population can be studied both directly and indirectly. The direct method tries to establish a statistical relation between the frequency of variants of a gene in the population (its alleles) and temperamental traits. This method provides estimates of the influence of specific genes on specific traits and requires molecular biological knowledge about which genes and alleles can be distinguished. This direct method became feasible towards the end of the twentieth century as the relevant knowledge became increasingly available.

The present article reviews results from familial analyses conducted in the field of behavioral genetics, a subdiscipline of population genetics (see Plomin et al. 1997 for an overview). This indirect method estimates the overall genetic influence on a specific dimension of temperament relative to the overall environmental influence on this dimension through analyses of the temperamental similarity of members of the same family such as twins, adoptive siblings, and their parents. This method requires only basic knowledge about rules of inheritance, not molecular biological knowledge about genes and their alleles. However, the method is based on certain assumptions that may not always be valid.

The indirect method is based on the fact that the genetic similarity of family members varies from 0 percent shared alleles in the case of adopted siblings or adopted children and their nonbiological parents, to 50 percent shared alleles in the case of biologically related siblings, nonidentical twins (dizygotic twins), and their biological parents, and 100 percent shared alleles in the case of identical twins (monozygotic twins). Siblings, nonidentical twins, and their parents share 50 percent of their alleles on average because half of children's alleles stem from the mother and the other half from the father. Identical twins are an exception because they develop from the same original cell that only later splits into two different individuals; they are therefore genetically identical.

Two basic assumptions of the indirect method are: (a) influences on temperamental differences in the population can be either environmental or genetic, and (b) the overall impact of the environment, relative to genes, is identical for siblings, adoptees, and twins. Therefore, differences in their similarity can be fully interpreted as genetic effects. Thus, if identical twins are more similar in a specific temperamental trait than nonidentical twins, or biologically related siblings are more similar than adopted siblings, this difference in similarity can be genetically interpreted. The larger the difference in similarity, the stronger the genetic influence on the temperamental trait. Thus, there exist two main indirect methods of estimating genetic influences: comparing identical twin pairs with nonidentical twin pairs (twin method), and comparing biologically related sibling pairs with adopted sibling pairs (adoption method). As a result it is possible to crosscheck the results of these two methods with one another. Other indirect methods compare the similarity between identical twins that are reared apart as opposes to those reared together, or the similarity between parents and their biological children as opposed to their adopted children.

The strength of the genetic influence on temperamental differences can be quantified by the heritability coefficient h2. This coefficient can be interpreted as the proportion of variance in a specific temperamental trait in the population that is due to genetic differences: h2=genetic variance/total variance. It can be estimated on the basis of differences between intraclass correlations. In the twin method, the intraclass correlation between nonidentical twins (a measure of their similarity) is subtracted from the intraclass correlation between identical twins. Because this difference estimates 100−50 PERCENT=50 percent of the genetic influence (identical twins share 100 percent of their alleles, and nonidentical twins 50 percent), it is doubled; this double difference is h2. In the adoption method, the difference between the sibling correlation and the adopted sibling correlation is doubled because it estimates 50−0 PERCENT=50 percent of the genetic influence (adopted siblings do not share alleles).

The heritability coefficient can vary between 0 and 100 percent. Zero heritability would be found if the variation of the temperamental trait in the population is only due to environmental differences. In this case, adopted and biologically related siblings are equally similar in this trait, as are identical and nonidentical twins. A heritability of 100 percent would be found if the variation of the trait in the population is only due to genetic differences. In this case, adopted siblings would be as dissimilar as children from different families, and identical twins would perfectly match each other in the trait; normal siblings would be moderately similar.

The concept of heritability is a relativistic concept because it estimates the amount of genetic influences relative to environmental influences. Therefore, heritability can vary between populations and between different traits within the same population. Thus, the heritability of adolescent shyness can be higher in the USA than in African cultures, and adolescent shyness can be more heritable than adolescent sociability in the USA. Furthermore, heritability depends on the variability of genes and environments in a given culture. If the genetic variability is increased (e.g., through genetically dissimilar immigrants), or if the environmental variance is decreased (e.g., through compensatory education), the genetic influence can increase; if the genetic variance is decreased (e.g., through a virus that kills people with particular alleles), or if the environmental variance is increased (e.g., through an unjust political system), the genetic influence can decrease.

The heritability of a trait depends not only on the trait, the population, and its culture, but also on age. The genetic influence on a trait may decrease with age because more and more environmental influences accumulate. On the other hand the genetic influence may also increase with age because more and more genes become relevant for the trait or because older children and adults are more able than young children to select their environment according to their genetically influenced interests and motives (such selection would decrease the environmental variance). All in all, heritability is a highly relativistic concept with regard to the target trait, culture, and age.

2.2 Nutrition

Even though genetic influences are an important determinant of individual variation in height, differences in average height across populations are almost entirely the product of environmental influences, not genetics. Although many environmental factors affect body stature, chief among them is nutrition. Of particular importance is nutrition in early life. Numerous scientific studies have shown that severely malnourished children have a greatly increased mortality risk relative to normal children (Behrman and Deolalikar 1988).

Infancy is the period of life in which both the human growth rate and the risk of mortality is at its highest. It is not surprising, therefore, that it is also the time during which nutritional intake is the most critical. Indeed, the concern for nutrition may begin in utero, and several studies have shown that supplementing the diet of pregnant women in developing nations significantly increases birth weights—the most important predictor of infant mortality. Even in developing nations, infant mortality is higher than any other period of life. By the age of two, much of the deficit characteristic of adults has already accumulated. Nutrition during the adolescent growth spurt may be important, though the evidence is mixed. In any case, the impact on morbidity and mortality of adolescent malnutrition is nowhere near as dramatic as it is in early childhood.

Many sub-populations in developing nations have mean birth weights that are similar to those in developed nations. In these developing nations, however, weight gain begins to diminish at about six months, the time when mothers' lactating ability declines and children are often weaned. Although food supplements can help, the prevention and treatment of infectious diseases is also critical in this period in developing nations.

2.2 Gene–Environment Covariation

The most reasonable explanation of the growing genetic influence on cognitive abilities across the life span is that the environments that contribute to individual differences in cognitive development become increasingly a function of the individuals' genotypes (Plomin 1994). The older children and adolescents become, the more they create and choose their environments, and they probably do so in ways such that their environments fit their genetically influenced dispositions. For example, more intelligent children tend to be more successful in intellectually demanding tasks and thus tend to persist with them, whereas less intelligent children experience more failure in those tasks and therefore turn to less demanding activities. This is sometimes referred to as a smörgåsbord view of the environment: The technical term for that is genotype–environment covariation of the evoked (or reactive) and active variety.

2 Molecular Genetic Associations with Personality Traits

The second major approach to the study of genetic influences on personality is to test whether specific genetic variants, or alleles, are associated with mean level differences in personality traits. Because personality traits are most often continuous measures and heritability analyses suggest that these traits are polygenic, the challenge is to find genes of medium effect size that influence personality levels. These genes are known as Quantitative Trait Loci or QTLs. Most efforts to identify which genes may be associated with personality traits are currently focusing on genes involved in neurotransmission, such as dopamine receptor genes and serotonin receptor and transporter genes. Many of the findings are only preliminary, and often with contrasting results or lack of replication in other studies or populations.

Finding genes that have relatively small effects will present great challenges to behavioral and genetic scientists, for several reasons. First, because many genes are likely to be involved, each with a small effect, very large sample sizes will be necessary to obtain sufficient statistical power. Second, many of the genes may be interacting with other genes, a phenomenon known as epistasis. For example, a particular allele for gene A may only influence personality negatively if a specific variant of gene B is present. Preliminary findings from large population-based studies suggest that some epistasis exists for personality and genes in the dopaminergic system. Third, there may well be substantial interactions between genes and environments that hamper the potential to find genotypic or even environmental associations. The impact of negative life events is greater in individuals with a genetic susceptibility to major depression, suggesting an interaction between genetic and environmental influences. Similarly, the effects of some allele may be enhanced or reduced depending on environmental circumstances. Again, large samples with genetic and detailed environmental characterization will be necessary to delineate the respective roles of genes and environments. In this fashion, scientists will come closer to understanding not only how much, but how genes influence personality.

2.3 Anxiety Disorders

Some behavioral disorders can phenomenologically be considered as extremes of behavioral variants with a broad variation in the general population. For example, anxiety is a complex behavioral reaction physiologically revealed in dangerous situations; another example is eating disorders (anorexia, bulimia) which might be considered as variants of dieting. The physiological reactions are expressed in an interindividually, quantitatively, and qualitatively variable manner under the same situational context. The interindividual variation of these behavioral traits is partly under genetic control, as evidenced by twin studies. The degree of genetic impact is variable across traits with strong effects on anxiety proneness and smaller effects on dieting. Thus, it did not come as a surprise that the phenomenologically related disorders demonstrate familial similarity and genetic influences. However, the magnitude of genetic influence may vary along the behavioral continuum, and a qualitatively additional effect might operate on the extremes (i.e., on the disorders). An additional genetic effect was indeed observed for anorexia whereas a qualitatively additional effect on anxiety disorders seems to be less likely.

Anxiety disorders display a phenomenologically heterogeneous and variable symptomatology overlapping with nearly all other psychological disorders. Subtyping of anxiety disorders is widely accepted on the basis of distinct phenomenological features. The various clinical variants (generalized anxiety disorders, panic disorders, phobias) reveal specific familial aggregation in family studies, although substantial intrafamilial cosegregation between various specific anxiety disorders and also with depression (especially generalized anxiety disorder but panic disorder substantially less so) and addictive behavior (panic disorder, phobias) was observed. The absolute prevalence rates for specific disorders vary considerably between studies due to methods of case identification and sampling. The reported relative risks vary between 2 and 10. Twin studies including multiple anxiety/affective disorders demonstrated that:

(a)

Generalized anxiety disorder, panic disorder and specific phobic disorders are under genetic influence (with heritability rates between 30 percent and 45 percent); obsessive-compulsive disorder seems to have the lowest level of genetic influence.

The genetic contribution to each anxiety syndrome is neither highly specific nor highly unspecific (partly syndrome-specific and partly shared by other anxiety syndromes and by unipolar depression).

Different anxiety disorders are genetically heterogeneous with at least two genetically distinct groups: panic disorder, phobias and bulimia defining a group of disorders with broad overlap of influencing genetic factors, and generalized anxiety and depression defining a separate genetically overlapping group (Kendler et al. 1995).

The phenotype transmitted in families is not only restricted to specific clinical syndromes. Increased anxiety proneness, behavioral disinhibition, increased sensitivity to hyperventilation or elevated autonomic reaction were also observed more commonly than expected by chance among healthy relatives (Merikangas et al. 1999).

The search for specific genes influencing anxiety disorders has been unsuccessful up to now. Some genome-wide linkage studies were performed for panic disorder without providing conclusive results on the localization of susceptibility genes. In any case, a major gene effect was not found. Thus, it is very likely that multiple genes influence the risk for panic disorder, each with an effect too small to be detected by linkage analysis. On the other side, associations with variants of candidate genes which are known to be involved in the pathophysiology of anxiety could also not be detected up to now (Van den Heuvel et al. 2000) (see Anxiety and Anxiety Disorders).

Research has shown that both genes and environment influence intelligence. As a result, behavioral genetic studies have begun to examine how these genetic influences operate developmentally and how genes affect the relationship among diverse cognitive skills such as verbal and spatial ability. In general, these studies have shown that genes account for roughly 50% of the variance in intelligence and that these genes are associated with stability across time and across task. As a result, attempts to find DNA markers associated with intelligence should expect that these markers will also be associated with this stability. Finally, behavioral genetic studies have also attempted to identify the environments that impact cognitive development.

2.3.1 Heredity

Although we know that antisocial tendencies such as aggression tend to run in families, we cannot say unequivocally whether this family effect is due to the common environment, or the genetic influences shared by the family members, or both (Geen 1998). The few investigations employing behavioral measures have obtained only weak, if any, indications of a hereditary patterning in the disposition to violence. By contrast, the Miles and Carey (1997) meta-analysis found that both heritability and family environment contributed to individual differences in personality measures of aggressive inclinations. This analysis also suggested that the relative importance of genetic influences increases with entry into adulthood.

Behavioral genetics includes both quantitative genetic and molecular genetic approaches to study behavioral differences between individuals. Human quantitative genetic studies include twin and adoption designs. Molecular genetic studies are beginning to identify specific genes responsible for genetic influence on behavior. Substantial genetic influence has been found on severe disorders such as schizophrenia, milder disorders such as anxiety and reading disability, and normal variation such as personality and cognitive abilities. Unlike single-gene disorders such as PKU, behavioral disorders and dimensions are likely to be influenced by many genes, called quantitative trait loci (QTLs).

2 Historical Context

The relative influence of nature and nurture on g has been studied since the beginning of psychology. Indeed in 1865, a year before the publication of Gregor Mendel's seminal paper on the laws of heredity, Francis Galton published a two-article series on high intelligence and other abilities, which he later expanded into the first book on heredity and cognitive ability, Hereditary Genius: An Inquiry into its Laws and Consequences (1992; originally published in 1869). The first twin and adoption studies in the 1920s also focused on g. Highlights in the history of genetic research on g include Leahy's (1935) adoption study, in which she compared IQ resemblance for nonadoptive and adoptive families. This study confirmed an earlier adoption study that showed genetic influence, in that IQ correlations were greater in nonadoptive than in adoptive families. The first adoption study that included IQ data for biological parents of adopted-away offspring also showed significant parent–offspring correlation, suggesting genetic influence (Skodak and Skeels 1949). Begun in the early 1960s, the Louisville Twin Study was the first major longitudinal twin study of IQ that charted the developmental course of genetic and environmental influences (Wilson 1983).

In 1963, a review of genetic research on g was influential in showing the convergence of evidence pointing to genetic influence (Erlenmeyer-Kimling and Jarvik 1963). During the 1960s, environmentalism, which had been rampant until then in American psychology, was beginning to wane, and the stage was set for increased acceptance of genetic influence on g. Then, in 1969, a monograph on the genetics of intelligence almost brought the field to a halt, because the monograph suggested that ethnic differences in IQ might involve genetic differences (Jensen 1969). Twenty-five years later, this issue was resurrected in The Bell Curve (Herrnstein and Murray 1994) and caused a similar uproar. The causes of average differences between groups need not be related to the causes of individual differences within groups. The former question is much more difficult to investigate than the latter, which is the focus of the vast majority of genetic research on IQ. The question of the origins of ethnic differences in performance on IQ tests remains unresolved.

The storm raised by Jensen's monograph led to intense criticism of all behavioral genetic research, especially in the area of cognitive abilities. These criticisms of older studies had the positive effect of generating a dozen bigger and better behavioral genetic studies that used family, adoption, and twin designs. These new projects produced much more data on the genetics of g than had been obtained in the previous 50 years. The new data contributed in part to a dramatic shift that occurred in the 1980s in psychology toward acceptance of the conclusion that g is significantly associated with genetic differences between individuals (Neisser et al. 1996).

4 Trait and Biological Approaches

In everyday life, people readily characterize each other in terms of personality characteristics: he or she is friendly, assertive, submissive, conscientious, and so on. The essence of the trait approach, whose fundamental premises date back to the ancient Greeks, is the assumption that behavior is primarily determined by a number of stable, generalized personality traits that express themselves in many contexts. Guided by this assumption, advocates of this approach try to identify and measure individuals' traits and to discover the most fundamental traits on which people can be compared.

A principal focus of research on traits is on measurement—that is, the development of quantitative ways of finding and describing important stable individual differences. Traits are inferred from questionnaires, ratings, and other reports about the person's dispositions. Usually, the person's self-reports (or someone else's reports about the person) are taken as direct signs of the relevant traits. For example, the more one rates oneself as aggressive, the more one is assumed to have an aggressive disposition. The trait approach recognizes that behavior can vary depending on the situation but has focused on individual differences in the overall response tendency averaged across many situations.

Some consensus has grown among many researchers to focus on five large factors or dimensions of personality that have emerged from statistical analyses of traits. These factors, often referred to as the ‘Big Five,’ comprise openness to new experience, conscientiousness, extraversion (or outgoingness), agreeableness, and neuroticism. Considerable stability has been demonstrated on trait ratings and questionnaires related to the Big Five, particularly during the adult years (McCrae and Costa 1990; see Extraversion; Neuroticism).

In a different direction, the British psychologist Hans Eysenck (1916–97) and his associates have led the way in connecting psychological dispositions to their biological foundations. According to Eysenck, introverts need only a small amount of stimulation to overstimulate their central nervous system (CNS) which then leads them to become withdrawn in their behavior. In extraverts, by contrast, the CNS is not easily stimulated, leading them to seek activities that will increase stimulation levels, for example, by socializing more actively and by seeking activities such as parties. In support of his theory, Eysenck (1971) found that extraverts reported earlier, more frequent, and more varied sexual experiences. In another study, introverts showed greater changes in their brain wave activity in response to low frequency tones (Stelmack and Michmaud-Achron 1985), indicating their lower threshold for stimulation to the CNS.